Testing the hypothesis. It’s often said that incumbent parties recover in the polls as a general election approaches – but is it actually true? I thought I’d sift through the wonderful resource that is Anthony Wells’ bank of historic opinion polls to find out. The above chart is what I came up with. It shows the difference between the governing party’s poll rating and the Opposition’s in the run up to all the elections since 1992. I’ve used MORI – later Ipsos MORI – data mainly because they’ve been polling across the entire period.

And the results are? I admit, it’s not the most scientific of methods: taking a single MORI poll from one year before an election, then six months, and then the election result itself. Yet it’s enough to draw some conclusions from. And the first of those conclusions? That incumbent parties do tend to rally across the final year of their incumbency. For instance, Labour went from being 13 points behind the Conservatives in May 2009’s Ipsos MORI poll to 7 points behind in the actual election. Even in the Labour landslide of 1997, the Tories came from 27 points behind to 12.

It’s not quite that simple (1) Come election time, the incumbent party tends to improve on its scores from a year earlier. What about from half a year earlier? In the last three elections, Labour’s final tally was, if anything, slightly worse than what the opinion polls were saying six months before. If David Cameron is counting on a late surge in Tory votes, this time around, he could be left disappointed. The surge, such as it was, might already have happened.

It’s not quite that simple (2) And then there’s the great political head-scratcher that is the 1992 election. John Major’s Conservative Party was equal with Labour in the MORI poll of April 1991. They were 6 points behind in the poll of October 1991. And yet they ended up with an 8 point lead in the election itself. What was that all about? There are many possible answers, but one of the most certain is that the pollsters simply got it wrong. They’ve all revised their methodology in the intervening years, mostly to account for past voting behaviour. Opinion polls are much more sophisticated nowadays.

Don’t expect another ’92. The opinion polls could be wrong again, just as Elvis could still be alive – but it’s a scant hope. If Cameron does pull off a majority in May, it will be because he’s managed to change voters’ minds in the short months between now and then; particularly in those crucial marginal seats. Not because we didn’t know where voter’s minds were at in the first place.